There are many industries producing fiber-reinforced resin composite parts. For instance, composite parts are commonly used in the automotive, marine, industrial, and aerospace industries.
Depending on the requirements of each industry, various methods and processes can be used to produce composite parts. One commonly known method is the resin transfer molding (“RTM”) process in which reinforcing materials (e.g. glass fibers, carbon fibers, etc.) are placed into a closed mold and then impregnated at high pressure (e.g. 400 psi and higher) with a liquid matrix (e.g. a polymer resin). In a variant of the RTM process, the closed mold is put under vacuum prior to the injection, at atmospheric pressure, of the matrix to impregnate the reinforcing materials. Such a process is generally known as a vacuum assisted resin transfer molding (“LIGHT RTM”) or (“VARTM”) process. In line with the VARTM process is the advanced VARTM (“A-VARTM”) process. In A-VARTM process, the mold is usually open and light weight compared to other RTM or VARTM processes. To compress the layers of reinforcement materials on a complex mold shape, a flexible vacuum bag is used. When the bag is put under vacuum, the atmospheric pressure insures the proper compaction of the reinforcing materials and removes air in the bag. After impregnation of the reinforcing fibers with the matrix, the pressure on the bag becomes neutral and degassing become difficult.
One of the problems of composite parts made from VARTM processes is the porosity. Indeed, despite due care, the impregnation of the reinforcing materials with the matrix is never perfect and the resulting composite part typically contains porosities such as voids and gas bubbles around fibers and in the matrix. Though porosities are generally not a major problem in the automotive and marine industries, they are a significant problem for the aerospace industry. Indeed, in the aerospace industry, the porosity content of a composite part must be severely controlled to prevent its failure.
Unfortunately, current RTM, VARTM, even A-VARTM processes are not able to produce composite parts with the requisite limited amount of porosities suitable for the aerospace market.
To overcome the shortcomings of VARTM processes, the aerospace industry currently produces composite parts using a specific process, sometimes referred to as pre-preg, using reinforcing materials pre-impregnated with a resin matrix and ready to be vacuum bagged and cured at high temperature (e.g. 130° C. and higher) in a pressurized autoclave. The main advantage of autoclaved pre-impregnated composite parts is the almost complete absence of voids and porosities (typically less than 1%). However, the pre-impregnated process is excessively expensive. For instance, pre-impregnated reinforcing materials must be stored at −18° C. or colder to slowdown the cure cycle of pre-mixed resin, they have to be thawed many hours before usage and they need significant supervision. In addition, the pre-impregnated process requires a pressurized autoclave which is very expensive, particularly for large composites parts.
Hence, there is a need for an improved A-VARTM process and associated molding apparatus which could mitigate at least some shortcomings of prior art VARTM processes and which could be able to produce composite parts and articles with a porosity level similar or better to the pre-impregnated process.